The invention relates to the field of permanent magnets, and more particularly to magnets of the ferrite type comprising the magnetoplumbite type.
The present invention relates to permanent magnets of the ferrite type based on the magnetoplumbite phase MFe12O19 with M=Sr,Ba,etc . . . , in which the element M is partially substituted by an element R chosen from among the rare earths or bismuth, and in which the element Fe is partially substituted by at least one transition metal T.
Such magnets are already known for their high magnetic properties, as disclosed in the Japanese patent application J10-149910 or in the European patent application EP-0 905 718 or in the international patent application WO99/34379.
In these patent applications, lanthanum La is generally used as the element M, and cobalt Co as element R.
Manufacture of such magnets comprises the following stages:
a) formation of a mixture of raw materials either by wet process to form a dispersion, or by dry process to form granules,
b) calcination of the mixture up to 1250xc2x0 C. to form a clinker, or fire clay, comprising the magnetoplumbite phase required, said mixture, under either dispersion form or granular form, being introduced into a calcination furnace.
c) wet grinding of the clinker until an aqueous dispersion of particles of around 1 xcexcm is obtained, under the form of a paste with about 70% dry extract
d) the paste is concentrated and compressed under an orienting magnetic field of about 1 Tesla and under a pressure of 30 to 50 MPa in such a way as to obtain an anisotropic xe2x80x9cgreen compactxe2x80x9d, with about 87% dry extract.
e) after drying and elimination of the remaining water, sintering of the green compact,
f) final machining to obtain the magnet of the defined shape.
The major problem of magnets according to the state of the art described above is their very high cost. In fact, the substitution elements La and Co are very expensive, lanthanum being much more expensive than strontium, and cobalt oxide being about 200 times more expensive than iron oxide.
In any case, the aim of the invention is therefore to increase the quality/price ratio of present magnets, either by increasing their performance, or by lowering their price, or by playing on the two factors simultaneously.
The aim of the invention is a method making it possible to achieve this, together with the magnets obtained by this method.
According to the invention, in the manufacturing method for permanent magnets of the ferrite type comprising a magnetoplumbite phase, formula M1xe2x88x92xRxFe12xe2x88x92yTyO19 in which M=Ba, Sr, Ca, Pb, and R=Bi or elements of the rare earth group, T=Mn, Co, Ni, Zn, and with x and y comprised between 0.05 and 0.5:
a1) one forms in a mixing means, typically a mixer operating by batch, a pulverulent mixture MP of raw materials relative to the elements M, R, Fe and T, typically under the form of oxide, carbonate, hydroxide, etc., comprising water in a predetermined quantity, typically needed for the formation of granules in the subsequent stage,
a2) next one transforms, typically in a granulator operating continuously, said mixture of xe2x80x9cgreen compactxe2x80x9d pellets A, with possible addition of supplementary water,
b) one calcinates said xe2x80x9cgreen compactxe2x80x9d granules to form a clinker B, with a magnetoplumbite base of formula M1xe2x88x92xRxFe12xe2x88x92yTyO19,
c) one carries out a wet grinding of said clinker, typically in an aqueous medium, to obtain a homogeneous dispersion C of fine non-agglomerated particles of average particle size lower than 1.2 xcexcm,
d) one concentrates and compresses said particles under an orienting magnetic field to form an anisotropic xe2x80x9cgreen compactxe2x80x9d D which can be manipulated, to the pre-determined shape,
e) said anisotropic green compact is sintered to obtain a sintered element E,
f) possibly, a final dimensioning of said sintered element is carried out, typically by machining, to obtain the final magnet F, the method being characterised in that:
1) at the stage a1) of the method, said mixture is formed, by introducing in said mixing means, a dry mixture MS of powders corresponding to said raw materials relative to the elements M and Fe and a homogeneous fluid dispersion DF of said raw materials relative to the elements R and T typically in said pre-determined quantity of water,
2) at stage b) of the method, said green granules A are calcinated at a chosen temperature and for a chosen length of time,
in such a way as to obtain, at the end of stage b), a clinker B which is both homogeneous in chemical composition and in size, and in apparent density, comprised between 2.5 and 3.5 and which is easy to grind in stage c).
The method according to the invention is clearly different from the methods according to the present state of the art. In fact, the applicant has been able to compare the methods according to the state of the art and those according to the invention, all other things remaining equal. It was noted that the differences of method led to great differences of final magnetic performance of the magnets and/or manufacturing cost. Different hypotheses are put forward below and the facts observed are interpreted as follows:
A) Comparison of the Method According to the Invention with the State of the Art Wet Process
In this state of the art method, a dispersion of raw materials is formed in water and then the dispersion formed is introduced directly into the calcination furnace, typically a rotary furnace operating continuously and, at the exit from the furnace, the clinker B is recuperated.
In this method, the chemical composition of the dispersion to be calcinated is very homogeneous, and thus also that of the clinker B obtained; on the other hand, the sizes of the clinkers obtained at the end of calcination is very heterogeneous, and typically extends from 0.5 mm to 10 mm.
A clinker size is considered to be homogeneous when the spread TMxe2x88x92Tm (difference between the maximum size and the minimum size) is lower than the average of these values (TM+Tm)/2.
The applicant has observed that, during the wet grinding of the stage c), it was impossible to grind this clinker to obtain a dispersion C of particles of homogeneous particle size and centred around 1 xcexcm, doubtless because the granules of small size are ground more quickly than the granules of large size, the ratio of sizes being typically between 1 to 20 and even more By using this method, the applicant has noted the formation of ultra-fine particles, namely of typical particle size lower than 0.3 xcexcm. These particles are known to pose several disadvantages:
on the one hand, the concentration at the stage d), typically by filtration, is expensive because of the high level of losses (the finest particles being carried away with the water) and because of the length of time of filtration or concentration, taking into account the presence of ultra-fine particles obstructing the filtration media,
on the other hand, the ultra-fine particles cannot be oriented by the orienting field, because their magnetic couple is too low, which results in a final magnet with remanence Br which is too weak.
furthermore, the time and the energy required for grinding these ultra-fine particles are pure losses in cost.
B) Comparison of the Method According to the Invention and the Dry Process According to the State of the Art
In the method according to the state of the art, the first stage a1) comprises the formation of a mixture of powders MP comprising all the constituents of the final magnet together with the major part of the quantity of water needed for the formation of the granules of stage a2), and then this pulverulent mixture MP is transformed into green granules A at stage a2) . In order to achieve this, a granulator is typically used in which one introduces, continuously, on the one hand the MP mixture of powders and, on the other hand, the quantity of supplementary water needed for the formation of the granules, while extracting the green granules A formed continuously. It can be observed that the granules A obtained, generally under the form of xe2x80x9cballsxe2x80x9d, have a relatively homogeneous size, ranging typically from 4 to 6 mm in equivalent xe2x80x9cdiameterxe2x80x9d. Furthermore, after calcination, one obtains a clinker B with approximately the size of the granules A introduced into the calcination furnace. Such a clinker makes it possible, with varying lengths of grinding time, to obtain a dispersion C and, after filtration, a paste whose particle size distribution is better than that obtained with the wet process, since it is narrower and better centred around about 1 xcexcm.
However, following these investigations, the applicant became convinced that, in the case in particular of magnets comprising partial substitution elementsxe2x80x94elements R and T, typically La and Co respectively, added in relatively low quantities, the magnets of the state of the art obtained by this method did not provide optimum properties.
In fact, within the framework of her work on the manufacturing methods, the applicant studied different ferrites and, in particular, measured the values of the field of anisotropy Ha, especially in function of the nature of the element M and the degree of substitution x and y of the ferrite, by the SPD (single point detection) method, such as described by Messrs. G. Asti and S. Rinaldi in the Journal of Applied Physics, Vol 45, No 8, August 1974, pages 3600 to 3610.
This method consists of measuring the magnetisation xe2x80x9cmxe2x80x9d of the clinker B in function of an applied field H, and of producing a graph of the curve d2m/dH2 in function of H: the field of anisotropy Ha is equal to the field H for which d2m/dH2 has a single point.
The applicant observed, as proved by the measurements carried out on the clinker B and on the magnets E or F of tests 1a to 6c, that this method makes it possible to reach the value of Ha, even from a raw magnetic material and practically isotropic such as a clinker.
The applicant noted, as will be seen in the examples, that the value of Ha obtained is practically the same, whether the latter is measured on a clinker B, relatively isotropic, or on a final magnet (E or F) obtained after sintering, relatively anisotropic, such that this value Ha is evidently an intrinsic property of a clinker of a given ferrite material, a value independent from orientation and particle size, and therefore from all the processes of the stages after stage b) for synthesis of the magnetoplumbite phase.
Thus, it seems that this value Ha is independent, once the magnetoplumbite phase has been formed, from the processes of grinding, compacting and sintering, and generally from the micro-structure and the particle size which may result from processes after stage b), which proves to be very useful for understanding the phenomena under study.
Furthermore, by comparing the values of Ha and the final magnetic properties (Br, HcJ and shape factor of a hysteresis cycle) for different materials and in function of different manufacturing processes for the clinker B, the applicant has made several observations and has presented the hypotheses leading to the present invention:
on the one hand, the applicant found that, in certain cases, and particularly in the case where the mixture of elements, in stage a), is made by the so-called wet process, the ferrites substituted by R and T of the state of the art showed final magnetic properties which were too weak, and not in relation to their relatively high values of Ha,
on the other hand, the applicant came to the conclusion that, for a given composition of a magnet, typically defined by M, R, T, x and y, it was the so-called wet process which made it possible to obtain the highest values of Ha, these values, measured on the clinker, flattening out as soon as the clinker B typically reached a density of at least 2.5.
The applicant has presented the hypothesis that, in these conditions, these maximum values of Ha obtained were the maximum intrinsic values characteristic of a given magnet composition, very close to the properties measured on a monocrystal of the same composition.
Consequently, the applicant considered that the comparison between the values of Ha measured on the clinker B obtained in a given process and the intrinsic values of Ha could be a measure of the chemical homogeneity of the clinker, or furthermore of the degree of transformation into the magnetoplumbite phase.
Thus, the applicant discovered that, particularly in the case where the formation of green granules A, in stage a), is carried out by the so-called dry process of the state of the art, the level of Ha of the clinker thus obtained was too low, taking into account the chemical nature of the elements M, R and T of the ferrite, compared to the intrinsic values of Ha of clinkers obtained by the so-called wet process, of the same chemical nature, and all other things being equal, especially the calcination conditions. Thus the applicant, after having put forward the hypothesis according to which the relatively low level of performance of ferrite magnets substituted by R and T could come from an insufficiently complete transformation of said mixture into the magnetoplumbite phase, the clinker obtained by the dry process not having a sufficiently homogeneous nor sufficiently rich chemical composition in the magnetoplumbite phase, next put forward another hypothesis and presumed that this could result from insufficient homogeneity of the chemical composition of the green granules A at the beginning, at their entry into the calcination furnace. The applicant explored this track, which led to modifying the method of stage a1), as mentioned above.
Continuing this work, the applicant also studied the calcination of green granules A obtained according to the invention, and observed that a sector of optimum density existed for the clinker obtained according to the invention, following the calcination stage.
In fact, she noted that, in the case of green granules according to the invention, the magnetic performances of the magnets were better when the apparent density d. of the clinkers B formed were comprised between 2.5 and 3.5.
To explain the existence of this area, the applicant put forward the following hypotheses, understanding that it is already known that the density of clinker rises with the temperature and the length of the stay in the calcination furnace:
on the one hand, above an apparent density of 2.5, it seems that the clinker has a heterogeneous chemical composition, and certainly too low a content in ferrite of the magnetoplumbite type, as if the formation reaction of the ferrite was incomplete,
on the other hand, above an apparent density of 3.5, the clinker becomes harder and it can be considered that, as a result of a longer grinding time to obtain the same average particle size, more ultra-fine particles are formed which, as already mentioned above, do not contribute to the final magnetic properties which, in the two cases, can contribute to limiting the final magnetic performances.
Thus, the applicant developed a process which cumulates the advantages of the two methods known in the present state of the art, without having the disadvantages, and which makes it possible to form a clinker with both a high homogeneity of size and chemical structure and with a high level of magnetoplumbite phase.
This method according to the invention is thus a combination of two means, one of which concerns the homogeneity of the chemical composition of the green granules A to be calcinated and therefore that of the clinker B formed after calcinationxe2x80x94the size or size-distribution of the green granules A being little modified during calcination, and the other of which concerns control of the calcination of the green granules A, not by the temperature itself but by the apparent density of the clinker B formed, these two means having been diagnosed as essential by the applicant, for improving the value of the anisotropy field Ha and the final magnetic properties of the magnets.
Since calcination furnaces typically operate continuously and there is a bi-univocal relation between the apparent density da of the clinker and the calcination temperature, itself a function of fuel consumption of the means of heating the furnacexe2x80x94typically a gas burner, it is simple to pilot and regulate the consumption of the burner according to the measurement of apparent density of the clinker at the exit from the calcination furnace.
It is well known that the measurements of temperatures in continuous rotary furnaces, carried out typically by optical pyrometers, are relatively imprecise, less as far as the measurement itself is concerned, than for the very heterogeneous temperature distribution in each of the portions of space or volume of the furnace. It is therefore very advantageous to be able to pilot a furnace without using this type of temperature measurement, even though temperature measurements are still used, in particular for verifying continuously and in real time the stability of operation of the furnace.
As will be shown in the comparative tests, the results obtained with the method according to the invention have shown a very clear improvement in the magnetic performance of magnets E and F, all other things being equal, which allows one to consider that the hypotheses of the applicant are valid without any doubt.